The present invention relates to a magnetic sensor for detecting a magnetic field and, more specifically, to a magnetic sensor using spin tunnel phenomena or spin valve effect.
1. Field of the Invention
With the recent increased density of the magnetic recording technique, relative speed between a magnetic medium and a reading head has been much lowered.
2. Description of the Related Art
Accordingly, the conventional electromagnetic induction type magnetic head has found it difficult to have sufficient reading output.
As a magnetic sensor which can have high reading output even with the lowered relative speed, a magnetoresistance (MR) magnetic sensor, a spin tunnel magnetic sensor using spin tunnel phenomenon, etc. are proposed.
The spin tunnel magnetic sensor comprises a multi-layer body of a magnetic layer/an insulation layer/a magnetic layer in which an insulation layer is sandwiched by two magnetic layers, and uses the phenomena that when a voltage is applied between the magnetic layers to tunnel electrons, a tunneling probability of electrons changes based on a relative angle between magnetization directions of both magnetic layers. A tunnelling probability of electrons changes based on a relative angle between magnetization directions between both magnetic layers because electron spin of one of the magnetic layers which supplies electrons is polarized, and the electrons tunnel, polarized.
In the conventional spin tunnel magnetic sensor, it is generally known that an insulation film is sandwiched between a first magnetic thin film and a second magnetic thin film and adhered to each other.
Japanese Patent Laid-Open Publication No. 24477/1994 proposes a magnetic sensor comprising a first ferromagnetic thin film and a second ferromagnetic thin film patterned in strips to make axes of easy magnetization rectangular to each other, whereby a coercive force of the first ferromagnetic thin film in a direction of the axis of easy magnetization thereof is more than twice that of the second ferromagnetic thin film in a direction of the axis of easy magnetization thereof. When the magnetization direction of the second ferromagnetic thin film having a smaller coercive force is turned by an external magnetic field, a tunneling current from the first ferromagnetic thin film to the second ferromagnetic thin film changes.
As a material of the ferromagnetic thin film, an Fe-based alloy, which has low anisotropic magnetoresistive effect and high ferromagnetic tunnel effect is proposed (Nakatani and Kitada, Abstract of Autumn Symposium of Japan Metal Association, p. 364, 1994).
Furthermore, to make a coercive force difference between the ferromagnetic thin films, carbon (C) or ruthenium (Ru) is added to the Fe-based alloy, or the thin films are formed at different substrate temperatures.
A magnetic sensor using a multi-layer thin film is known as a different spin tunnel magnetic sensor.
Japanese Patent Laid-Open Publication No. 266481/1991 proposes a magnetoresistive effect device comprising a multi-layer structure of Fe layers with an intermediate layer of a paramagnetic non-insulating material. The device exhibits resistance changes with respect to a low-level applied magnetic field by making magnetization directions horizontally anti-parallel with each other and making the Fe layers in four or more layers.
Japanese Patent Laid-Open publication No. 74022/1995 discloses a magnetic head using a magnetoresistive effect film of a multi-layer structure including a hard magnetic layer, a soft magnetic layer which contacts an antiferromagnetic layer, a soft magnetic layer which is not in contact with an antiferromagnetic layer which are laid one on another, respectively, through non-magnetic layers. The magnetic sensor exhibits high magnetoresistive effect because of the multi-layer body including the two magnetic layers.
Japanese Patent Laid-Open Publication No. 223336/1994 proposes a magnetoresistance read sensor comprising first, second and third ferromagnetic layers which are separated from each other by non-magnetic metal layers. Magnetization directions of the first and the third ferromagnetic layers are stationary, and the second intermediate ferromagnetic layer is soft magnetic material and has a multi-layer double spin valve structure in which, when no magnetic field is applied, a magnetization direction thereof is rectangular to the magnetization directions of the first and the third ferromagnetic layers. This structure permits conduction electrons scattering in any direction to be used, so that the sensor exhibits high magnetoresistive effect even when a low-level magnetic field is applied.
A magnetoresistive effect device of a magnetic sensor using a magnetoresistive effect comprises a spin valve film having a structure of a non-magnetic layer sandwiched by first and a second magnetic layers, or a superlattice gigantic magnetoresistance (GMR) film having a structure of alternate layers of non-magnetic and magnetic material.
Conventional magnetoresistive effect devices will be explained with reference to examples thereof, respectively, including a spin valve film and a superlattice (GMR) film as the MR films.
FIG. 20 is a sectional view of the spin valve film of the conventional magnetoresistive effect device according to one of the examples, and shows the spin valve film of the magnetoresistive effect device used in a magnetic head.
As shown in FIG. 20, the conventional spin valve film has a structure of a first magnetic layer 23, a non-magnetic layer 25, a second magnetic layer 27 laid one on another on a substrate 21 with a ground layer 22 deposited on, and an antiferromagnetic layer 28 of, e.g., FeMn for pinning a magnetization direction of the second magnetic layer.
FIG. 21 is a sectional view of the superlattice GMR film of the conventional magnetoresistive effect device according to the other of the examples, and shows the superlattice GMR film of the magnetoresistive effect device used in a magnetic head.
As shown in FIG. 21, the conventional superlattice GMR film has a structure of a multi-layer film of alternately laid magnetic layers 23 and non-magnetic layers 25 on a substrate 21 with a ground layer 22 of Cu, and a cap layer 29 of Cu covering the top surface of the uppermost magnetic layer 23.
A magnetic sensor using GMR effect is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 358310/1992. this magnetic sensor comprises two ferromagnetic layers which are divided by a non-magnetic metal layer and are not bonded with each other, and has a sandwich structure having magnetization of one of the ferromagnetic layers pinned. The pinning of the magnetization is enabled by adhering an antiferromagnetic metal layer of typically an iron-maganese alloy to one of the ferromagnetic layers. In this structure, when an external magnetic field is applied, a magnetization direction of the ferromagnetic layer whose magnetization is not pinned freely turns in agreement with a direction of the external magnetic field, whereby an angle difference takes place with respect to magnetization direction of the ferromagnetic layer having the magnetization pinned. Depending on this angle difference, scattering of conduction electrons depending on spin changes, and electroresistance value changes take place. By detecting such electroresistance value changes, states of an external magnetic field, i.e., signal magnetic fields from a magnetic recording medium are obtained.
The resistance change of the spin valve magnetic sensor is about 5%. For the prevention of reading errors due to increased magnetic recording density, magnetic sensors having higher magnetic resistance changes are needed.
Furthermore, a magnetic bead and a recording medium are often brought into direct or indirect contact with each other due to projections of the magnetic recording medium, dust or others. At points of the contact abrupt temperature rises occur due to frictional heat. It is known that due to such temperature changes, a resistance of the MR device changes, and output changes take place. Such output changes are called thermal asperities or thermal noises. Conventional art for removing such asperities is described in Japanese Patent Laid-Open Publication No. 154310/1990. This art comprises two MR devices, and the two MR devices are differential for differential detection, whereby thermal asperities are canceled.
As described above, various magnetic sensors using spin tunnel phenomena have been proposed. However, their voltage changes due to spin tunnel phenomena are trivial, and, in addition, signals from recording media are increasingly feeble. Then, it is necessary increase outputs of the magnetic sensors, and to decrease noises.
In the magnetic sensor comprising the spin valve film shown in FIG. 20, the non-magnetic layer 25 is formed of, in most cases, a Cu layer, which produces high magnetoresistive effect. However, a magnetic material of the magnetic layers 23, 27 is an alloy containing an element which tends to be solid-soluble with Cu, e.g., Fe, Co or Mn. Accordingly, thermal diffusion tends to occur in the interfaces between the magnetic layers 23, 27, and the non-magnetic layer 25, and the thermal diffusion tends to be caused by a heat treatment of a magnetic head fabrication process following deposition of the spin valve film, e.g, by baking a resist used as an insulating layer, which often reduces magnetoresistive effect.
Also in the magnetic sensor comprising the superlattice GMR film shown in FIG. 21, the magnetic layer 23 and the non-magnetic layer 25 are formed of, in most cases, magnetic layer containing an element which tends to react with Cu and a non-magnetic layer of Cu, and, accordingly, tend to cause thermal diffusion in a heat treatment process as in the spin valve film, which often leads to lower magnetoresistive effect.
To improvise heat resistance of such spin valve film and superlattice GMR film, it is proposed that the non-magnetic layer is formed of Bag in place of Cu. However, the non-magnetic layer of Ag is aggregated by a heat treatment when the Ag layer is thin, and the spin valve film and the superlattice GMR film are sometimes broken. When the Ag layer is made thicker for the prevention of the aggregation, the magnetoresistive effect is decreased. Thus, it is difficult that the magnetoresistive effect device comprising the non-magnetic layer formed of Ag has high magnetoresistive effect.
Furthermore, Cu tends to corrosive so that it is difficult to provide a magnetic sensor comprising the non-magnetic layer formed of Cu having good corrosion resistance in use environments.
As described above, in the conventional magnetoresistive effect device comprising the non-magnetic layer formed of Cu, a diffusion reaction takes place between the non-magnetic layer and the magnetic layer contacting the non-magnetic layer, which often reduces the magnetoresistive effect. Disadvantageously is reduces the magnetoresistive effect to form the non-magnetic layer of Ag for the prevention of the thermal diffusion. The magnetoresistive effect device comprising the non-magnetic layer formed of Cu has insufficient corrosion resistance.
By making the spin valve magnetic sensor differential, output improves about two-fold, and cancellation of thermal asperities can be expected, but the method described in the prior art needs a track width for two MR devices, which does not meet narrow tracks for higher recording densities.